1. Field of the Invention
The invention relates to a novel process for incorporating CaF.sub.2 into a polyamide.
2. Description of the Prior Art
It is known from British Pat. No. 1,211,685, that polyamides having CaF.sub.2 incorporated therein as a nucleating agent can be melt spun into filaments at higher spinning temperatures and at a higher extrusion rates than when the CaF.sub.2 is omitted from the polyamide. It is further known from this patent that the CaF.sub.2 is incorporated into a polyamide by injecting an aqueous CaF.sub.2 slurry thereof into the salt solution during the polymerization process (e.g. during the second cycle) by which the polyamide is produced or by melting polyamide flake which has been coated with CaF.sub.2. According to the patent CaF.sub.2 is prepared by the well-known process of double-decomposition of suitable calcium and fluoride salts in aqueous solution at or near the boiling point. Upon mixing the salts, CaF.sub.2 precipitates in the aqueous medium and may be recovered therefrom as a paste or slurry such as by filtration or centrifugation followed by decantation.
U.S. Pat. No. 3,755,221 relates to fast cycling polyamide molding resins and suggests that instead of adding an aqueous CaF.sub.2 slurry to the salt solution, the CaF.sub.2 may be prepared in the salt solution.
U.S. Pat. No. 3,529,929 relates to an improvement in the double-decomposition process described in British Pat. No. 1,211,685 whereby CaF.sub.2 of finer particle size is prepared by bringing together and rapidly mixing the aqueous solutions at a temperature ranging from 0.degree. to 60.degree. C.
One drawback associated with incorporating CaF.sub.2 into a polyamide by the above-mentioned prior art techniques is that such techniques tend to cause foaming of the resulting molten polyamide/CaF.sub.2 mixture which in turn causes processing difficulties, and in many instances renders further processing impossible. Another drawback is that the prior art techniques tend to result in agglomeration of CaF.sub.2 in the polyamide. Another drawback associated with the prior art techniques is that the CaF.sub.2 is first prepared by a rather tedious process involving numerous purification steps to remove any by-products which would inhibit polymerization of the polyamide if present during the polymerization process.
Accordingly, it is an object of the present invention to provide a simple process for incorporating CaF.sub.2 into a polyamide which avoids the above-mentioned drawbacks of the prior art processes.